The callsFake() Function in Sinon
Sinon stubs have a callsFake() function that tells Sinon what function to call instead of the stubbed function.
For example, you can replace the axios.get() function with a fake function as follows.
const axios = require('axios');
const assert = require('assert');
const stub = sinon.stub(axios, 'get').
callsFake(() => Promise.resolve({ status: 200 }));
// Calls the fake `axios.get()`
const test = await axios.get('https://httpbin.org/get');
test.status; // 200
test.data; // undefinedThe callsFake() function is handy for testing, because you can configure the behavior of any function call to handle hard-to-test code paths.
For Async Functions
To stub async functions, we typically recommend making your fake function return a promise using Promise.resolve().
Making sure your fake function returns a promise is especially important if you are using promise chaining, because otherwise you won't be able to call then().
const stub = sinon.stub(axios, 'get').
callsFake(() => Promise.resolve({ status: 200 }));You can also pass an async function to callsFake().
However, that can be indicative of a code smell, because fake functions typically should not do anything besides returning a pre-defined value.
Multi-line fake functions are typically unnecessary.
However, you may use the following syntax instead of Promise.resolve() because the following is more concise.
const stub = sinon.stub(axios, 'get').
callsFake(async () => ({ status: 200 }));For Errors
You can also make your fake functions throw errors for testing error cases. For example, you can make a stub return a rejected promise as follows.
const axios = require('axios');
const assert = require('assert');
const stub = sinon.stub(axios, 'get').
callsFake(() => Promise.reject(new Error('Oops!')));
// Calls the fake `axios.get()`
try {
await axios.get('https://httpbin.org/get');
} catch (err) {
err.message; // Oops!
}You can also throw an error from the fake function body. Just make sure you're consistent about async functions vs sync functions: if you're stubbing an async function, make sure you either return a promise or use an async fake function!
// Good: fake returns a promise
sinon.stub(axios, 'get').
callsFake(() => Promise.reject(new Error('Oops!')));
// Good: fake is async
sinon.stub(axios, 'get').
callsFake(async () => { throw new Error('Oops!'); });
// Bad: fake throws sync error, even though axios.get() never does
sinon.stub(axios, 'get').
callsFake(() => { throw new Error('Oops!'); });